Re: [PATCH] serial: core: Fix atomicity violation in uart_tiocmget

[Ilpo Järvinen]

On Fri, 12 Jan 2024, John Ogness wrote:

> On 2024-01-12, Gui-Dong Han <2045gemini@xxxxxxxxx> wrote:
> > In uart_tiocmget():
> >     result = uport->mctrl;
> >     uart_port_lock_irq(uport);
> >     result |= uport->ops->get_mctrl(uport);
> >     uart_port_unlock_irq(uport);
> >     ...
> >     return result;
> >
> > In uart_update_mctrl():
> >     uart_port_lock_irqsave(port, &flags);
> >     ...
> >     port->mctrl = (old & ~clear) | set;
> >     ...
> >     uart_port_unlock_irqrestore(port, flags);
> >
> > An atomicity violation is identified due to the concurrent execution of
> > uart_tiocmget() and uart_update_mctrl(). After assigning
> > result = uport->mctrl, the mctrl value may change in uart_update_mctrl(),
> > leading to a mismatch between the value returned by
> > uport->ops->get_mctrl(uport) and the mctrl value previously read.
> > This can result in uart_tiocmget() returning an incorrect value.
> >
> > This possible bug is found by an experimental static analysis tool
> > developed by our team, BassCheck[1]. This tool analyzes the locking APIs
> > to extract function pairs that can be concurrently executed, and then
> > analyzes the instructions in the paired functions to identify possible
> > concurrency bugs including data races and atomicity violations. The above
> > possible bug is reported when our tool analyzes the source code of
> > Linux 5.17.
> >
> > To address this issue, it is suggested to move the line
> > result = uport->mctrl inside the uart_port_lock block to ensure atomicity
> > and prevent the mctrl value from being altered during the execution of
> > uart_tiocmget(). With this patch applied, our tool no longer reports the
> > bug, with the kernel configuration allyesconfig for x86_64. Due to the
> > absence of the requisite hardware, we are unable to conduct runtime
> > testing of the patch. Therefore, our verification is solely based on code
> > logic analysis.
> >
> > [1] https://sites.google.com/view/basscheck/
> >
> > Fixes: 559c7ff4e324 ("serial: core: Use port lock wrappers")
> 
> It fixes c5f4644e6c8b ("[PATCH] Serial: Adjust serial locking").

That commit only extracted the locks from ->get_mctrl() into the caller
but this assignment was outside both pre and post that commit (the issue 
goes all the way back into history.git domain into 33c0d1b0c3eb ("[PATCH] 
Serial driver stuff") which introduced ->mctrl).

-- 
 i.

> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Gui-Dong Han <2045gemini@xxxxxxxxx>
> > ---
> >  drivers/tty/serial/serial_core.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> > index 80085b151b34..a9e39416d877 100644
> > --- a/drivers/tty/serial/serial_core.c
> > +++ b/drivers/tty/serial/serial_core.c
> > @@ -1085,8 +1085,8 @@ static int uart_tiocmget(struct tty_struct *tty)
> >  		goto out;
> >  
> >  	if (!tty_io_error(tty)) {
> > -		result = uport->mctrl;
> >  		uart_port_lock_irq(uport);
> > +		result = uport->mctrl;
> >  		result |= uport->ops->get_mctrl(uport);
> >  		uart_port_unlock_irq(uport);
> >  	}
> 
> Looking over the RMW accesses to @mctrl, I expect you will also need
> this hunk:
> 
> @@ -2242,6 +2242,7 @@ uart_set_options(struct uart_port *port, struct console *co,
>  {
>  	struct ktermios termios;
>  	static struct ktermios dummy;
> +	unsigned long flags;
>  
>  	/*
>  	 * Ensure that the serial-console lock is initialised early.
> @@ -2279,7 +2280,9 @@ uart_set_options(struct uart_port *port, struct console *co,
>  	 * some uarts on other side don't support no flow control.
>  	 * So we set * DTR in host uart to make them happy
>  	 */
> +	uart_port_lock_irqsave(port, &flags);
>  	port->mctrl |= TIOCM_DTR;
> +	uart_port_unlock_irqrestore(port, flags);
>  
>  	port->ops->set_termios(port, &termios, &dummy);
>  	/*
> 
> FWIW,
> Reviewed-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
>